The present invention relates to a fixing device and an image forming apparatus.
In a conventional image forming device of an electric photography type such as a printer, a copier, and a facsimile, a fixing device of a thermal roller type is provided for fixing a toner image transferred to a sheet. Such a fixing device is provided with a fixing roller as a fixing member and a pressing roller. When the sheet passes through a nip portion formed between the fixing roller and the pressing roller, a toner image is fixed to the sheet.
In a fixing device of an endless belt type, an endless belt (fixing belt) is provided between a heating roller and a fixing roller as a fixing member, so that the fixing roller is pressed against a pressing roller with the fixing belt in between.
In the fixing device of the belt type, as compared with the fixing device of the thermal roller type, it is possible to reduce power consumption. Further, it is possible to reduce a period of time necessary for achieving a state capable of fixing from an idle state, that is, to reduce a period of time necessary for increasing a temperature. This is because a belt has a relatively small heat capacity, and it is easy to control a temperature. However, due to the small heat capacity, a temperature tends to drop rather quickly during fixing. To this end, a heater may be provided in each of the heating roller and the pressing roller (for example, refer to Patent Reference).
Patent Reference: Japanese Patent Publication No. 2002-196604
In the conventional fixing device, the nip portion is formed between the fixing roller and the pressing roller. The fixing roller is formed of a material with a large heat capacity such as a silicone rubber. Still, when sheets pass through consecutively, the fixing roller tends to lose heat very rapidly. To compensate the heat loss, the fixing roller is heated through thermal control. However, it is difficult to quickly heat a surface of the fixing roller due to the large heat capacity and low responsiveness. On the other hand, when the fixing roller is heated continuously for a long period of time, the fixing roller may be heated too high, thereby causing a fixing problem due to a high temperature.
In view of the problems described above, an object of the present invention is to provide a fixing device and an image forming apparatus capable of solving the problems. In the fixing device, it is possible to prevent a temperature of a fixing member from increasing rapidly, thereby preventing a fixing problem.
Further objects and advantages of the invention will be apparent from the following description of the invention.